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Typical TDE width?

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#1 derangedhermit

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Posted 22 November 2012 - 10:06 AM

I'm making a square out of corrugated plastic with a cutout the size of my mirror, to sit just above the mirror, to better pull the air from the rear fan across the mirror's surface. I forgot the name of the person whose name is associated with this, or I would mention him. Anyway, I thought I'd make one or more similar pieces, each with a smaller cutout, to evaluate TDE. I figure I might as well since the signmaker would only sell me a 4x8 sheet of black corrugated plastic, so I have plenty of material.

Are there any typical numbers I can use to size the cutout? Is the TDE usually in the last 1/4", the last 1/2", the last 1", the last 10%? Or can it vary a lot, so that I need to make several rings of different sizes?

#2 Pinbout

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Posted 22 November 2012 - 10:40 AM

if you cut the hole the smallest you can progessively get larger to test the mirror and then fine tune the size [if there is a tde] with a new piece.

so if you start at 2" smaller, 1" all around, you can make it larger in half inch increments.

coroplast may be a little hard to cut in 1/4in increments. use fresh blades.

#3 Darren Drake

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Posted 22 November 2012 - 11:26 AM

If you place a knife edge at the focal point in the focuser while looking at a centered bright star you can see exactly where the tde is or even if there is one as well as any other optical defects.

#4 Mark Harry

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Posted 23 November 2012 - 07:31 AM

Put it on a foucault tester, and ascertain the -exact- width of the TDE, if any. Use the principle of seeing the diffraction line all the way around the edge of the mirror, with equal, or darn near equal brightness both sides.
M.

#5 ed_turco

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Posted 23 November 2012 - 03:24 PM

+1

#6 derangedhermit

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Posted 23 November 2012 - 06:52 PM

Put it on a foucault tester, and ascertain the -exact- width of the TDE, if any. Use the principle of seeing the diffraction line all the way around the edge of the mirror, with equal, or darn near equal brightness both sides.
M.

I wish I had the tester and the skill to use it - but not badly enough to start learning. Since I don't, I translate this in my case as "send it to someone who knows what they're about". Still not a bad idea.

#7 derangedhermit

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Posted 23 November 2012 - 06:54 PM

if you cut the hole the smallest you can progessively get larger to test the mirror and then fine tune the size [if there is a tde] with a new piece.

so if you start at 2" smaller, 1" all around, you can make it larger in half inch increments.

coroplast may be a little hard to cut in 1/4in increments. use fresh blades.

An excellent suggestion that I can implement. Cutting circles in Coroplast is trying. Rectangles are easy enough.

#8 Pinbout

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Posted 23 November 2012 - 07:04 PM

but you can do how darren said, you could lay a piece of tape across half of the focuser and defocus a bright star like vega.

eye placement is easier at focus than on a bench at center of curvature if you never did it before.

Dick Parker eludes to a KE test on a star in his testing optics vid

check out the images of the ke tests. they can be very revealing.

check out the spot test. look at the third image, it has a bright ring around the mirror, thats the diffraction ring, meaning no tde. but all those rings are zones which isnt so good a lot. :o

#9 dave brock

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Posted 23 November 2012 - 09:03 PM

check out the spot test. look at the third image, it has a bright ring around the mirror, thats the diffraction ring, meaning no tde.


Ahh but those images are all of the same mirror and the 1st image definitely shows a TDE. You can see theTDE in the 2nd image as a shadow out at the edge. Looks to me like the diffraction ring doesn't apply with the spot test.

Dave

#10 Pinbout

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Posted 23 November 2012 - 10:19 PM

thanks for the proper correction. :lol:

#11 Mark Harry

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Posted 24 November 2012 - 08:04 AM

My 2 cents, the spot test is an extremely -BAD- example of a test, on account of the mirror used. There is no excuse for having an 8" F/5 mirror look that bad of a sphere!!
And the pics have definitely been doctored IME. I would take anything written there as subject to review, and to proveability.
M.

#12 ed_turco

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Posted 24 November 2012 - 11:44 AM

TDE happens when any maker, including Me gets careless, but I deal with it. I've ground 1/64" off some edges because TDE is so injurious to the images.

I met a man with a 16" telescope with a 1" TDE painted over. I knew my place and held my tongue.

But I felt bad...

#13 Pinbout

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Posted 24 November 2012 - 11:49 AM

There is no excuse for having an 8" F/5 mirror look that bad of a sphere!!



it probably just came off his machine from fixed post polishing. :o

#14 Mark Harry

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Posted 24 November 2012 - 05:40 PM

Possible, but I think it would have been better to use a better example, especially when he 'publishes' such information.
M.

#15 Pinbout

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Posted 24 November 2012 - 09:30 PM

Did you go back a couple pages on his site. he shows a caustic test or lateral wire test.

I never seen that on the net or even explained. :shrug:

I guess I was searching caustic when I should've been looking for lateral wire test. :foreheadslap:

from this site

Replace the slitless KE w/t plastic w/t a vertical scratch (virtual wire). place the tester inside of ROC. Align the testers lateral axis perpendicular to the OA (optical axis). Mark the mirror w/t sharpy (or use an Evert stick) at the zones. Move the tester laterally so as to align the shadow of the virtual wire over the marked zone and record the lateral measurement.

LWT is a moving source test (virtual wire is above the source i.e., reflected & source ray are coincident).



:question:

#16 Mark Harry

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Posted 25 November 2012 - 07:11 AM

The best way (and most sensitive) to test a spherical surface is with a null at ROC- no grating, spots, wires.
Errors in small fractions of 1/100th wave are achieveable.
(IME,/IMO, but it's generally used by the majority of good mirror makers out there, and pretty well documented as well.) Edge problems are easily identified, also. If a star/planet test doesn't tell me anything else that I haven't found out already by this proven test, then I refrain from "trying to re-invent the wheel", so to speak.
My 2 cents, but it works for -ME-.
M.

#17 Pinbout

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Posted 25 November 2012 - 08:45 AM

The best way (and most sensitive) to test a spherical surface is with a null at ROC- no grating, spots, wires.



of course your right, it's the easiest way to measure the TDE

It really shouldn't take him that long to throw a stand together. He could hang it from a sling.


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#18 ed_turco

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Posted 25 November 2012 - 12:05 PM

I wish to gently say it, but you really need to learn the Foucault Test. It is a test that one has to learn, but in form it is so simple and elegant.


Ed

#19 kfrederick

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Posted 25 November 2012 - 06:14 PM

Well said ED. The knife edge all that is needed to make a great Newt primary .

#20 derangedhermit

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Posted 27 November 2012 - 04:58 PM

Thanks for the tips. I will investigate the Foucault test. I'm sure there are plenty of posts here and in other places about it.






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